Memory devices are used in a wide range of fields. For example, solid-state memory can be used for long or short term storage of program instructions and data in connection with computing devices. Memory density, access speed, and reliability are all important considerations in designing solid-state memory. Recent solid-state memory designs include resistance change memories, such as resistive random access memory (ReRAM), and phase-change random access memory (PCRAM or PRAM). These can be incorporated into three-dimensional architectures. Such designs can increase memory density. However, in a resistance change memory using an existing access transistor, a floor area per unit cell is increased. As a result, it is difficult to achieve increased memory density as compared to flash memories. However, it is possible to reduce a floor area per unit cell in a resistance change memory where the memory cells are disposed in a cross-point memory array configuration. In such a configuration, memory cells are provided at the cross points of intersecting wiring lines. Accordingly, an increase in memory density can be achieved.
In a memory cell of a cross-point memory array, a selection device or switch device for cell selection is provided in addition to the memory device. Examples of a switch device include a PN diode, an avalanche diode, a switch device configured with use of a metal oxide, and a switch device that is switched at a certain threshold voltage by Mott transition to abruptly increase a current. However, because of constraints on the threshold voltage at which the switch devices are switched, and because a leakage current during non-selection can be large, obtaining an adequate threshold voltage for a resistance change memory device using existing switch devices is problematic.
As an alternative, a switch device can incorporate a chalcogenide material, such as an ovonic threshold switch (OTS). An OTS device is characterized by a current that increases abruptly at a switching threshold voltage or higher. This makes it possible to obtain a relatively large current density when the switch device is placed in a selection (ON) state. Moreover, a microstructure of an OTS layer made of a chalcogenide material is amorphous, and it is therefore possible to form the OTS layer under room-temperature conditions using a physical vapor deposition (PVD) method or a chemical vapor deposition (CVD) method.